A high proportion (9 out of 11 strains) of cultured skin fibroblasts from patients with mucolipidosis II (I-cell disease) possess abnormally high intracellular levels of free cystine and in this respect mimic, at least superficially, the condition encountered in cells from individuals with cystinosis, a hitherto unrelated disease. In contrast, all of four strains of cells from patients with the biochemically similar, but clinically less severe, mucolipidosis III (pseudo-Hurler polydystrophy) were found to possess essentially normal intracellular levels of this amino acid. Results of amino acid analysis indicated that, as with cystinotic cells, the elevation of cystine seen in I-cell fibroblasts is unique to that amino acid and does not simply reflect a generalized increase in total intracellular amino acid content. These observations appear to strengthen the notion that a defective lysosomal function may constitute the underlying abnormality in human cystinosis.